If this part is true, in this instance, I like it (even if the new guy is just meh). Armstrong is only 29 and there are signs that he could be very good in the future. His teaching of fundamentals seems to need improvement (or his staff needs to improve on that front), but we were not schematically in ridiculous positions last year. We know the difference by watching Grantham for years doing extreme, and at times dumb, stuff. Armstrong was fairly sound, with obvious execution issues (think of the long Utah pass or the many missed tackles).
If he can learn the "coaching" part of the job more than just the scheme concepts over a year, he could turn into a very good DC and keeping him on would not have caused any problems because he is so young and they hired his prior mentor.